1. Field of the Invention
The present invention relates to a power transmission device, particularly to a power transmission device for a work vehicle, which is configured to transmit torque from a driving source to a wheel.
2. Background Art
Driving devices, described in Publication of U.S. Pat. No. 6,491,600 and Japan Laid-open Patent Application Publication No. JP-A4105-193373, have been provided as exemplary driving devices for work vehicles such as motor graders. The devices include a hydraulic motor and a power transmission device. The power transmission device includes a two-stage planetary gear reducer and is configured to transmit torque of the hydraulic motor to a wheel.
FIG. 1 illustrates a schematic diagram of the driving device described in Publication of U.S. Pat. No. 6,491,600, In FIG. 1, a reference numeral “CL” indicates a rotary axis. In the driving device, torque from a motor 1 is inputted into a sun gear 2 of a first stage. Further, a carrier 3 of the first stage is coupled to a sun gear 4 of a second stage, while a ring gear 5 of the first stage is coupled to a carrier 6 of the second stage and a wheel. Yet further, a ring gear 7 of the second stage is coupled to a stationary housing 9 through a clutch mechanism 8.
On the other hand, FIG. 2 illustrates a schematic diagram of the driving device described in Japan Laid-open Patent Application Publication No. JP-A-1105-193373. In the driving device, torque from the motor 10 is inputted into a sun gear 12 of a first stage through a pair of reduction gears 11a and 11b. Further, a carrier 14 of the first stage is coupled to a sun gear 15 of a second stage, while a ring gear 16 of the first stage is coupled to a ring gear 17 of the second stage. The ring gears 16 and 17 are also coupled to a wheel through a clutch mechanism 18. Yet further, a carrier 19 of the second stage is coupled to a stationary housing 20.
FIG. 1 depicts a torque transmission path in the device of Publication of U.S. Pat. No. 6,491,600 with arrows. Torque inputted in the sun gear 2 of the first stage is herein combined with torque from the ring gear 5 of the first stage, and the resultant torque is inputted into the sun gear 4 of the second stage through the carrier 3 of the first stage. Torque inputted into the sun gear 4 of the second stage is combined with torque from the ring gear 7 of the second stage, and the resultant torque is transmitted to the carrier 6 of the second stage. Torque reaction force of the ring gear 7 of the second stage is applied to the stationary housing 9 through the clutch mechanism 8. A part of the torque transmitted to the carrier 6 of the second stage drives the wheel, while the rest of the torque (see a path depicted with a broken line in FIG. 1) is returned to the planetary gear reducer of the first stage as the reaction force of the aforementioned ring gear 5 of the first stage. In short, torque is circulated through the device described in Publication of U.S. Pat. No. 6,491,600. Due to this, relatively large torque is required for driving the wheel. Especially, members forming the planetary gear reducer of the second stage are herein required to have a durable structure against such relatively large torque. Consequently, the device is inevitably formed in a large size.
On the other hand, in the device of Japan Laid-open Patent Application Publication No. JP-A-H05-193373, the input side of the clutch mechanism 18 is coupled to the ring gears 16 and 17, while the output side thereof is coupled to the wheel. Therefore, the clutch mechanism 18 is required to couple or decouple rotatable members. Now, the clutch mechanism 18 is generally formed by members such as a piston configured to be actuated by means of hydraulic pressure. In the device of Japan Laid-open Patent Application Publication No JP-A-H05-193373, members such as a piston are disposed on the rotatable members. In this type of structure, oil for actuating the piston is required to be supplied from a stationary side portion to the rotatable members. However, it is difficult to seal between the stationary side portion and the rotatable members. Therefore, a complex structure is required for preventing oil leakage from the part.
it is an object of the present invention to achieve efficient torque transmission and a seal structure of a hydraulic circuit continued to a clutch mechanism with a simple structure.